Timothy
Johnson
*a,
Magdalena M.
Łozińska
b,
Angelica F.
Orsi
b,
Paul A.
Wright
b,
Sheena
Hindocha
a and
Stephen
Poulston
a
aJohnson Matthey Technology Centre, Blount's Court, Sonning Common, RG4 9NH UK. E-mail: timothy.johnson@matthey.com
bEaStCHEM School of Chemistry, University of St Andrews, Purdie Building, North Haugh, St Andrews, Fife, KY16 9ST, UK
First published on 1st October 2019
The ability to produce large scale quantities of MOF materials is essential for the commercialisation of these frameworks to continue. Herein we report how the production of ZIF-94 can be improved from a ∼1 g laboratory preparation to a scalable procedure allowing for large scale production of the desired framework. The synthesis of ZIF-94 was completed at room temperature, atmospheric pressure and without the use of DMF as a solvent. This method offers improvements over the current literature synthesis routes and affords a product at 18 wt% solids. To demonstrate the robustness of the derived methodology a 60 g, large scale, batch of this framework was produced which possessed a surface area of 468 m2 g−1. This large scale sample has superior CO2 uptake of 3.3 mmol g−1 at 1 bar, an improvement of 30% over literature reports.
This paper presents a case study for the scale-up of MOFs. Here the synthetic methodology of a zeolitic imidazolate framework (ZIF), a subclass of the MOF family of materials, was studied and improved. This will show how small, low concentration, reactions can be built upon to produce materials in a scaleable and green fashion.
ZIF-94 is a framework composed of zinc tetrahedral metal ions connected by the organic linker 4-methyl-5-imidazolecarboxaldehyde in a sod topology, Fig. 1. ZIF-94, also known as SIM-1 (Substituted imidazolate material), is of interest due to its excellent chemical stability and high CO2 adsorbance at low pressure.15
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Fig. 1 (a) ZIF-94 linker 4-methyl-5-imidazolecarboxaldehyde and (b) representation of ZIF with a sod toplogy. |
Previous reports regarding the synthesis of ZIF-94 consist of very dilute solutions, generally between 5–12 wt% solids in DMF, under solvothermal conditions.5,15–18 When dealing with the scale-up of this material, these factors present a logistical barrier due to the large solvent volumes and the need for large reactors. In addition, for applications in gas separation membranes, the particle size of the MOFs utilized is an important factor. For improved processability sub <100 nm particle sizes are optimal however reports of MMM using sub-micron particles have been reported.19
Recently we reported how nano particles of ZIF-94 could be produced in a green, scalable fashion. This route utilised methanol and tetrahydrofuran as solvents. While methanol is considered a “recommended” solvent, tetrahydrofuran is “problematic”.20 This should be viewed in the context that, prior to work conducted by the authors, the production method to produce this material required dimethylformamide, a solvent with a “hazardous” rating due to its health effects. This signifies a significant improvement especially considering this material may be produced industrially at large scale. Importantly previous reports produced material using very dilute reaction conditions of 1.4 wt% solids.21
4-Methyl-5-imidazolecarboxaldehyde and zinc acetate, the linker and metal source for ZIF-94, are currently commercially available. Importantly 4-methyl-5-imidazolecarboxaldehyde is a good candidate for production using mechanochemical routes,22 suggesting this can be produced with minimal environmental impact and zinc acetate, being a salt, can be precipitated from aqueous conditions – further reducing the use of organic solvents.
ZIF-94 also offers excellent scope for post synthetic modification (PSM) due to the abundance of aldehyde groups within the pores of the material. The use of post-synthetic modification is of commercial interest since targeted functionality can be introduced into materials.
For ZIF-94, the addition of a long, hydrocarbon chain primary amine has demonstrated improvements in N2/CO2 separation under humid conditions5 and in Knoevenagel condensations23 where the MOF was used as the catalyst.
Herein we report the scale-up and production of a 60 g batch of ZIF-94. The procedures were specifically chosen to ensure scalability which includes the greening of solvents, room temperature synthesis and improved reaction concentration. This results in reduced demand for solvents and energy, while ensuring a pure nano-ZIF is produced.
Entry | Solid content (wt%) | Reaction time (min) | Zinc acetatea (g) | MeOH (mL) | THF (mL) | Mass of product (g) | Est. yieldc (%) |
---|---|---|---|---|---|---|---|
a The amount of linker (g) is equal to the amount of zinc acetate (g). b Mass not reported due to unintentional loss during transfer. For an explanation of how solid content was calculated, please see ESI.† c Estimated yield obtained from TGA data. | |||||||
1 | 0.3 | 15 | 1.07 | 228 | 571 | — | — |
2 | 0.3 | 60 | 1.07 | 228 | 571 | — | — |
3 | 0.3 | 360 | 1.07 | 228 | 571 | — | — |
4 | 4 | 30 | 2.64 | 40 | 100 | 3.18 | 86.9 |
5 | 4 | 60 | 2.64 | 40 | 100 | 3.12 | 85.7 |
6 | 4 | 180 | 2.64 | 40 | 100 | 3.4 | 93.9 |
7 | 4 | 960 | 2.64 | 40 | 100 | 3.43 | 95.2 |
8 | 4 | 960 | 1.32 | 20 | 50 | —b | — |
9 | 9 | 960 | 2.64 | 20 | 50 | 3.74 | — |
10 | 18 | 960 | 5.28 | 20 | 50 | 7.49 | — |
11 | 18 | 960 | 52.8 | 200 | 500 | 74.51 | 84.7 |
The zinc acetate dihydrate (52.8 g, 0.24 mol) was dissolved in 200 mL methanol. A separate solution of 4-methyl-5-imidazolecarboxaldehyde (52.8 g, 0.48 mol) was dissolved in 400 mL tetrahydrofuran. The methanol solution was added to the tetrahydrofuran solution under vigorous stirring. The mixture was continuously stirred at room temperature for 960 minutes. The product was collected by centrifugation (10 minutes, 11000 rpm) and washed with methanol three times. The resulting sample was dried at room temperature in air for 48 hours.
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Fig. 2 XRD patterns collected on samples synthesised at 0.3 wt% solid. Entries 1–3 (Table 1). |
Importantly it can be seen that at short reaction times, 15 minutes or less, an impure sample is produced. The impurity observed can be indexed to ZIF-93 – a framework produced from the same linker and metal as ZIF-94 but possessing the rho topology. Simulated PXRD patterns for both ZIF-94 and ZIF-93 can be seen in S1. Fig. 3 shows SEM micrographs which demonstrate that, with a reaction time of 15 minutes, poorly defined nano particles are produced. It is only when reaction times reach in excess of 30 minutes that nano spheres start to form. It is also of note that with a reaction time of 360 minutes nano particles continue to be observed.
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Fig. 3 SEM micrographs of samples produced at 0.3 wt% solids at (a) 15, (b) 60 and (c) 360 min respectively, Entries 1–3 (Table 1). |
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Fig. 4 XRD patterns collected on samples synthesised at 4 wt% solid (entries 4–7, Table 1). |
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Fig. 5 SEM micrographs of samples produced at 4 wt% solids at (a) 30, (b) 60, (c) 180 and (d) 960 min respectively. Entries 4–7 (Table 1). |
N2 isotherms collected on samples obtained from entries 4 and 8 (Table 1), S3 and 4, show BET surface areas of 432 and 571 m2 g−1, respectively. These values are comparable to literature values of 415 m2 g−1.18 This indicates that reaction time does not adversely affect the surface area of the framework produced and shows that this method is comparable to the use of low concentration reactions in DMF under solvothermal conditions.
Due to excess solvent within the pores of the material estimated yields have been calculated from the TGA data S2. It can be seen how estimated yields for these materials range from 85–95%. While these data show the yield increases with reaction time, most of the product is formed very quickly. This suggests that the formation of ZIF-94 is under thermodynamic control, with the kinetic product, ZIF-93, being formed initially and very quickly. These data indicate that with extended reaction time, equilibrium can be reached and the favoured thermodynamic product formed.
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Fig. 6 XRD patterns of samples produced at (a) 4 wt% (b) 9 wt% (c) 18 wt% respectively produced with reaction times of 960 min. Entries 8–10 (Table 1). |
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Fig. 7 XRD pattern and subsequent Le Bail fit for large scale ZIF-94, produced at 18% solids with a reaction time of 960 min, entry 11 (Table 1). |
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Fig. 8 SEM micrograph of ZIF-94 produced at 18% solids with a reaction time of 960 min, entry 11 (Table 1). |
TGA, S6, shows that the large scale batch of ZIF-94 contains a proportionately larger amount of trapped solvent than the smaller scale batches. A mass loss of 10% was observed before thermal decomposition of the framework occurred at 230 °C. This observation indicates that the drying procedure, which was adequate at small scales, did not transfer to larger scale experiments. A more rigorous drying procedure will be required in future, for example heating the framework below the thermal decomposition temperature in a vacuum oven.
Thermal decomposition progressed to 28% original mass, this is in line with theoretical value of 27% decomposition based on formation of ZnO and full linker destruction. The estimated yield observed for this reaction is also high at 84.7%. This is lower than that of past samples however this is likely due to the increased number of centrifuge pots needed during the separation – ultimately increasing transfer error. This yield is however still significant and, with improvements to separations, well within acceptable limits for large scale reaction procedures.
N2 isotherms, S7, were collected on the scaled up ZIF-94 sample. Subsequent BET analysis revealed the material possesses a surface area of 415 m2 g−1 which agrees with literature values as well as with samples produced at lower solid weight percent.18
The method of production reported herein utilises a centrifuge to separate the product from the mother liquor. It should be noted that this technique is a viable separation technique and used industrially to produce material at scale.27
CO2 adsorption data, Fig. 9, was also collected for this sample with a maximum uptake of 3.5 mmol g−1 observed. Importantly this large scale material outperforms previous literature reports of ZIF-94 which show CO2 uptakes of 2.5 mmol g−1 at 1 bar for samples produced hydrothermally in DMF, compared to 3.3 mmol g−1 at 1 bar for this work.15 This equates to a 30% performance improvement – increasing the validity of this material, and this synthetic method, for carbon capture applications.
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Fig. 9 CO2 adsorption isotherm collected at 0 °C for scaled up ZIF-94 sample, entry 11 (Table 1). |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9gc00783k |
This journal is © The Royal Society of Chemistry 2019 |